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Rectangular Shafts.

421

The Strength of a Rectangular Section was given by
St. Venant as follows :

Modulus of resistance of rectangular section = '294 4 -7^ — -
while the pure ring theory would give    *i666 b h ^W-^TP, and

the discrepancy increases with the ratio T
Thus if £=i"and h=*2n

Tm=*5266 ton ins. (i) by St. Venant; and 745 ton ins.

[(2) by ring theory,
and (i) — 7 of the diagram value (2).

If   £=i" and & = 4"
Tm = 1*142 (i) and 2747 (2) respectively
or (i) = '41 of diagram value.

Hexagonal shafts may be treated directly from diagram.

Strength  of Shafts by   Direct   Experiment. — The

following experimental figures may be used by way of correction.
Moment of any shaft d^ or s = Figure in table x (d* or s3).

BREAKING MOMENTS OF SHAFTS i" DIA. AND i" SQUARE,
IN TON-INCHES (.ELSWICK EXPERIMENTS).


	Round.
	Square.

Wrought Iron ..... .    ......
	T^c;
	6 -81

Cast Iron ........... .
	D Oj •\"?I
	uo 6*78

Steel ..................
	8-02
	11*6

Yellow Brass  ............
	2 'AC
	^*i <

Cast Copper   . . .....
	* *rj
 2'ic;
	o A 0 2'74


	
	

A factor of 10 is to be used for short shafts and of 16 for long
shafts, to secure stiffness. Strength is rarely the sole criterion.

Example 17.—Find the relative weights of two shafts of equal
strength; the one solid, and the other hollow, with a hole half the out-
side diameter. (Eng. Exam. 1892.)